1. Field of the Invention
This invention relates to a continuous process for bonding and stretching a fibrous polyolefin nonwoven sheet. In particular, the invention concerns such a process wherein the sheet temperature is varied during the stretching. When the bonding and stretching are performed without such temperature variation, the resultant sheet is significantly less uniform in thickness than sheet prepared in accordance with the present process.
2. Description of the Prior Art
Processes for manufacturing fibrous nonwoven sheets from polyolefin polymers are well known in the art. For example, steuber, U.S. Pat. No. 3,169,899 discloses depositing flash-spun plexifilamentary strands of polyethylene film fibrils onto a moving receiver to form a nonwoven sheet. Methods for assembling fibers deposited from a plurality of positions onto a moving receiver are disclosed by Knee, U.S. Pat. No. 3,402,227 and Farago, U.S. Pat. No. 4,537,733.
Several methods are known in the art for bonding and stretching fibrous polyolefin nonwoven sheets. A particularly useful method, especially suited for use in making lightweight nonwoven sheets of polyethylene plexifilamentary film-fibril strands, is disclosed by Lee, U.S. Pat. No. 4,554,207. Lee discloses a process that includes (a) forming a sheet of flash-spun, polyethylene plexifilamentary film-fibril strands, (b) lightly consolidating the thusly formed sheet, (c) heating the sheet without significant stretching to a temperature that is in the range of 3.degree. to 8.degree. C. below the melting point of the polyethylene, (d) then, while maintaining the sheet at that temperature, stretching the sheet in at least two stages to at least 1.2 times its original length and (e) finally, cooling the heated-and-stretched sheet to a temperature of less than 60.degree. C., preferably by first cooling through one surface of the sheet and then through the opposite surface. At substantially all times when the sheet temperature is 100.degree. C. or higher during the heating, stretching and cooling steps, forces are applied perpendicular to the surface of the sheet to restrain transverse shrinkage of the sheet. The process of Lee is illustrated with the simultaneous bonding and stretching of a fibrous polyethylene nonwoven sheet by passage over a series of heated rolls which reduces the unit weight of the sheet by as much as a factor of two.
The aforementioned methods have been technically useful and commercially successful in the manufacture of wide nonwoven sheets, particularly of polyethylene plexifilamentary film-fibril strands (e.g., "Tyvek" spunbonded olefin, manufactured by E. I. du Pont de Nemours & Co.). However, sheet uniformity problems are encountered in the known manufacturing processes, especially when lightweight sheets are made. Thin and thick areas are sometimes encountered in the lightweight sheets.
An object of the present invention is to provide an improved process for making a bonded-and-stretched fibrous polyolefin sheet that has improved thickness uniformity, even in very light unit weights.